Many things are not ready yet and they just picked the BD "excuse" for marketing/PR reasons. Cell is ready but the Nvidia chip has to go/is going through another (minor and last) correction round. All the crucial OS choices have been made after lengthy decision processes (to their credit they gathered a lot of inputs) but they still needed time to tune some and they have reverted a few lately (not that they had to but for the better).
Also current game alphas look good a la 360 while a few tech demos (trying to squeeze the max out of the cell/gfx) have been shown very recently and supposedly (we're entering a subjective world) they look better with even some whispering that after all some 1080 games may be possible. If this is true, it's another incentive for Sony to try to have some games available at launch that show a small but nonetheless noticeable tech advantage over the 360.
Finally rumours are circulating that they are pushing really hard to get 65nm Cells from IBM and 80nm Nvidia parts as early as possible. Still not in time for the launch, but earlier than anticipated so that they can release a PS3 v2 with lower production costs around mid 2007. If you factor the power/heat issues of the early 360s and the fact that the latest PS3 dev kit has a 1kW PSU and a zillion (very noisy) fans, it's possibly another incentive for the push.

Sony are very ambitious with the PS3 and IMHO the delay reflects both that ambition and their enthusiasm/optimism rather than terrible execution problems.